The many advantages of vapor phase chlorination of polyolefins over chlorination in solution are obvious and has resulted in substantial work in this area. While there are advantages to the vapor phase process, there are different problems in the vapor phase process. These include, particularly in fluidized beds, deposit of polymer on reactor walls, unsatisfactory mixing of the reactants, problems with heat transfer and polymer agglomeration resulting in channeling and ultimately collapse of the fluidized bed. Prior art processes are described below.
An attempt to eliminate problems in fluid bed processes for chlorinating polyethylene is described in U.S. Pat. No. 2,928,819 wherein the use up to 80%, preferably 50%, by weight of magnesia, aluminum sulfate, sodium chloride and the like is proposed, but the use of such large amounts of inorganic materials also introduce handling and other problems, including separation of these materials from the polymer and are not satisfactory for a commercial process.
U.S. Pat. No. 3,163,631 describes a process for halogenating a polyolefin in a finely divided state at an elevated temperature in the presence of an anti-static agent. The halogenation is achieved preferably by means of two stages: a partial halogenation at a temperature below the temperature of the onset of the endothermal fusion phenomenon characteristic for the treated polymer, in the presence, or better, in the absence of the anti-static agent, followed by intensive halogenation at a temperature such as defined above, in the presence of an anti-static agent that is a quaternary ammonium salt. This allegedly prevents degradation of the polymer during the reaction, and char formation at high temperatures. The patent teaches that during the reaction, the polymer often adheres to the walls of the reaction vessel but in the form of a very crumbly and readily detachable cake.
U.S. Pat. No. 3,887,533 is directed to chlorination of polyolefins in the solid state in a fluidized or moving bed. Thermostable chlorinated polyolefins are said to be obtained in accordance with this invention by reacting pulverulent polyolefins with gaseous chlorine in a fluidized layer or moving bed, in the presence of 0.01 to 3.0 weight percent of certain saturated aliphatic, aromatic, aliphatic-hydroxi-carboxylic and aromatic hydroxicarboxylic acids, alkali metal or alkaline earth metal salts thereof, and anhydrides thereof. The patent teaches that the process may be conducted in the presence of talc, silica gel, barium sulfate, polyvinyl chloride, and polyfluorohydrocarbons.
Two later U.S. Pat. Nos. 4,029,862 and 4,197,386 suggest using in the fluidized bed polyolefins having certain combinations of defined parameters including flowability, bulk density, surface area, density, melt index, hexane extractable waxes, etc., to obtain alleged improved processes and products.
British Patent Specification No. 1,415,236 discloses a method for the low temperature post chlorination in the dry state of polyvinyl chloride (PVC) in the presence of pigments known for use in subsequent compounding of the chlorinated PVC. Carbon black and titanium dioxide are listed as such compounding pigments for chlorinated PVC. These are used in the chlorination process in amounts as little as 0.001 part per 100 parts of resin. The examples show use of 0.02 parts of carbon black, 0.01 part of titanium dioxide, and use of 0.01 part of carbon black and 0.05 part of titanium dioxide together at a chlorination temperature of 50.degree. C.
However none of these processes is completely satisfactory and an improved process for fluid bed chlorination of polyolefins such as polyethylene that eliminates deposits on reactor walls, provides improvement in mixing in the bed, that has good heat transfer, minimum agglomeration and results in an improved uniformly chlorinated polyolefin with improved processing and physical characteristics is still desired.